The present invention relates generally to high pressure air guns of a type utilized for seismic exploration in marine environments, and more specifically relates to a high pressure air gun having improved air flow control passages to facilitate optimal air discharge and to yield optimal repeatability in air gun firing.
As is well-known, sources of acoustic energy are utilized in marine seismic exploration to create acoustic waves that propagate from the source into the marine floor. These acoustic waves contact the floor and subfloor geologic formations and are reflected back to hydrophones or other appropriate receiving instruments which receive the reflected pressure waves and convert them into electric signals which are then recorded. Analysis of the electronic signals facilitates evaluation of the structure of subsurface geologic formations.
Many devices have been utilized for generating these acoustic waves. In recent times, a device commonly utilized is an "air gun" which may be utilized to disperse any suitable compressed, gaseous fluid, such as air, steam, nitrogen, carbon dioxide, etc. The air guns are capable of releasing high pressure air on the order of 2,000 psi to 6,000 psi in the water to create the desired acoustic waves.
Conventional air guns typically include an annular housing which defines a chamber in which compressed air is stored, and selectively openable exhaust ports which allow the stored air to escape from the housing. These exhaust ports are selectively openable in response to the movement of a shuttle valve which, when shifted, permits air to escape from the chamber through the exhaust ports in the main housing into the surrounding water.
Conventional shuttle valves include a movable member which engages seal members on a housing assembly, with the shuttle valve member being slidable across the seals to open the exhaust ports. The frictional forces presented by the seal assemblies can present a significant opposition to initial, opening, movement of the shuttle valve member. This resistance presents significant operational difficulties relative to air guns.
A primary concern in the operation of air guns in marine seismic exploration is repeatability, or consistency, in timing of the opening of the air gun. In many applications, the opening time (or firing time) of the air gun must be consistent within plus or minus one millisecond. With this standard for timing repeatability, the effects of frictional forces on the shuttle valve member can easily cause an air gun to operate out of standard. This is true particularly because when conventional air guns are "at rest", or in the closed position, before the seal to the primary chamber is broken, the energy available to break that seal and to accelerate the shuttle valve member is limited to that which may be applied through a single piston area on the shuttle valve member. There is a need, therefore, to increase the initial acceleration of the shuttle valve member from the "at rest", closed, position, without requiring dramatic modification of basic air gun design.
Accordingly, the present invention provides a new method and apparatus for increasing the shuttle valve opening force and acceleration, and thereby improving the timing reliability of the opening of the shuttle, (the air gun firing time), resulting in improved repeatability of firing time performance.